1. Technical Field
The invention generally relates to transducers. More particularly, the invention relates to an audio transducer capable of reproducing a sound wave and having the benefits of planar and cone-type transducers.
2. Related Art
Various types of transducers are used to reproduce sound. Audio transducers may convert electrical energy into mechanical energy, such as the acoustical output from an audio loudspeaker. Audio transducers also may convert mechanical energy into electrical energy, such as the current output from a microphone. In the voice coil of a loudspeaker's transducer, an electrical audio signal from an amplifier interacts with a magnetic field of a stationary magnet to vibrate a diaphragm. If the vibration frequency is in the audible range, a sound is produced. In general, there are two types of transducers: cone-type (or dome-type) transducers and planar transducers.
Cone-type transducers have a cone usually made from paper, polymer, metal, or a combination of these materials. In a cone-type transducer, a cone is used to excite sound waves in a fluid such as air. The cone may be connected at its outer perimeter to a frame (usually of metal), by a pliable surround—a surrounding support of pliable material. The pliable surround is typically made of foam, rubber, or doped cloth. The inner perimeter of the cone may be connected to a tube structure (usually referred to as a former), which may be wrapped at the end opposite the cone with insulated wire to form a voice coil. Similarly, dome-type transducers use dome-shaped structures (instead of cone-shaped transducers) to excite sound waves. The voice coils of dome-type transducers, however, are typically produced using designs and techniques similar to those used with cone-type transducers.
For cone-type (and dome-type) transducers, the voice coil typically resides in a magnetic gap—a region where the stationary magnet produces a magnetic field. In cone-type transducers, the magnetic gap is generally constructed as a space inside the body of a stationary magnet structure, with the stationary magnets' field oriented orthogonal to the flow of current in the voice coil. The voice coil may be held so that the voice coil does not contact the walls of the stationary magnet.
The magnetic gap in a cone-type transducer is generally configured as a space that separates a magnetic north pole only slightly from a magnetic south pole. Thus, a voice coil placed within this space may be immersed in a relatively strong magnetic field. This relatively strong magnetic field enhances the efficiency of the transducer, better allowing the transducer to convert the power from an electrical signal into the mechanical power of a vibrating diaphragm.
When an electric current is applied through the wire windings of the voice coil in cone-type transducers, the current's interaction with the magnetic field generates a force on the voice coil that is perpendicular both to the magnetic field and to the direction of the current. Depending on the polarity of the electric potential applied to the voice coil, this force may move the voice coil deeper into or further out of the magnetic gap. This in and out movement of the cone causes the cone to vibrate and produce a sound wave.
In other words, when a time-varying electrical current corresponding to a sound wave is driven through a voice coil of a cone-type transducer, the current interacts with the field of the stationary magnet to vibrate the diaphragm. Thus, the diaphragm vibrates in response to the input electric potential. In this manner, the cone-type transducer can reproduce a sound wave that corresponds to the time-varying electrical current.
The distance that the cone moves into and out of the magnetic gap is referred to as excursion. Longer excursion lengths are helpful for providing a lower frequency response for the transducer and a greater acoustic output. Because the voice coil of a cone-type transducer moves in the magnetic gap, the stationary magnet structure subjects the voice coil to a substantially homogenous magnetic field throughout the excursion length. This benefit of a transducer design is described as “magnetic linearity.”
Cone-type transducers are typically characterized by a relatively high cone and coil mass, which limits the ability of the cone to vibrate at high frequencies. Some designs reduce the mass of the cone, but may do so at the cost of rigidity of the cone. Cones that are less rigid may suffer from distortion caused when a cone flexes instead of imparting pressure to the adjacent air. Flexing of the cone leads to “break-up”—a failure of the cone to properly reproduce a sound wave. Break-up may occur when the force applied to a cone excites a mechanical flexing mode of the cone instead of a motion that transmits the force into the adjacent air. While there is always some frequency at which a particular transducer cone will break up, a greater ability of the cone to resist flexing generally leads to a wider range over which the transducer may be used without distortion.
Planar transducers are different from cone-type transducers, both magnetically and mechanically. In a planar transducer, a planar diaphragm surface is used to excite sound waves in a fluid. Two common types of planar transducers are electrostatic and planar-magnetic transducers, which use electrical and electromagnetic forces, respectively, to vibrate a diaphragm.
In a planar-magnetic transducer, a diaphragm may be connected at two or more portions of its outer perimeter to a frame. The connection is typically made with an adhesive, but may also be made by fasteners or other mechanical connections. Unlike in a cone-type transducer where a pliable surround connects the diaphragm to the frame, a rigid attachment (usually by adhesive) is generally preferred in a planar transducer. This allows the diaphragm to be held under tension to prevent the diaphragm from sagging and contacting other components during operation.
The diaphragm generally has one or more voice coils integrated onto its planar surface, which are in the same plane as the diaphragm. Multiple stationary magnets are offset to the voice coils, with one or more of their poles generally directed toward the plane of the diaphragm.
The diaphragm of a planar transducer, which serves the same air-movement function as the cone of a cone-type transducer, is generally flat in comparison with the cone of a cone transducer. In a planar transducer, the break-up point of the diaphragm may be determined by the rigidity of the diaphragm material, the tension applied to the diaphragm, and the uniformity of the force applied to the back of the diaphragm. In a cone-type transducer, the break-up point depends on the rigidity of the cone material and the angle of the cone. Thus, with identical material rigidity, the breakup frequency of a cone-type transducer may be determined by cone angle while the breakup frequency for a planar transducer may be determined by diaphragm tension and by how evenly the movement force is applied to the diaphragm.
Both cone-type and conventional planar transducers present users with various disadvantages. For example, even though planar transducers can be significantly thinner than cone-type, planar transducers are unsuitable for many applications where their thinner structure would be a significant benefit. For example, planar transducers may require an impedance-matched transformer to match the impedance of the transducer to the amplifier.
While cone-type transducers may in some cases be more efficient and less complex than planar transducers, they are generally much thicker than planar transducers. Some cone-type transducer designs reduce the depth of the transducer, resulting typically in reduced performance. Some designs use a “cone” that is largely flat, thus reducing the depth of the overall structure. However, as the cone loses its angular orientation between its outer and inner perimeters, it looses structural rigidity. As the angle between the outer and inner perimeters of the cone approaches flat, the rigidity of the cone material must increase markedly. Other designs move the former, voice coil, and magnet to the interior or mouth of the cone. While this reduces the depth of the overall structure, distortion occurs as the sound wave generated by the vibrating cone deflects off the surfaces of the former, magnet, and frame structure.